Piezoelectric materials are generally metal oxides having a perovskite-type structure called perovskite-type metal oxides (ABO3). Lead zirconate titanate (hereinafter referred to as “PZT”) containing lead is a typical piezoelectric material including a perovskite-type metal oxide and is used in a variety of piezoelectric devices, such as actuators, vibrators, sensors, and filters. However, PZT contains lead of which the influence on environment is viewed as a problem. Accordingly, there is a demand for a piezoelectric material not containing lead (hereinafter referred to as “lead-free piezoelectric material”).
A solid solution of sodium niobate (NaNbO3) and barium titanate (BaTiO3) (hereinafter referred to as “NN-BT”) is known as an example of the lead-free piezoelectric material. The NN-BT ceramic does not contain potassium causing sintering resistance and low moisture resistance and therefore hardly causes a change with time in piezoelectric properties. In addition, when the NN-BT ceramic is used in a piezoelectric device, phase transition of the crystal structure does not occur in the operation temperature range (for example, 0° C. to 80° C.) of the device, and therefore the performance does not significantly vary by the operating temperature.
Technologies relating to NN-BT ceramics are described in, for example, PTL 1 and PTL 2.
PTL 1 discloses that a high piezoelectric constant is achieved by adding cobalt oxide (CoO) to an NN-BT ceramic.
PTL 2 discloses that a high piezoelectric constant is achieved by adding at least one element selected from Mn and Ni to an NN-BT ceramic.